The present disclosure relates to the construction of the stator of a progressive cavity pump or motor, and a method of assembly therefor.
Progressive Cavity (PC) pumps and motors have been used for years. The principle of operation of PC pumps was first described by Rene Moineau in his 1931 thesis and has since been known as the Moineau principle. Typically a PC pump or motor consists of a rigid helical rotor in a double-lead helical cavity stator. The differences in the leads between the rotor and the stator form cavities that progress axially from one end of the stator to the other as the rotor turns, moving the fluid through the pump or motor.
The stator is conventionally made of an elastomeric or plastic insert housed into a typically metallic, rigid, sleeve-shaped outer tube. The helical profile of the stator insert is typically formed by injection moulding the elastomeric or plastic material into the outer tube around a core. During normal operation of a PC pump, the rotor operates in tight contact with the stator insert, generating a high torsional force between the rotor and the stator. Accordingly, a tight bond is required between the stator insert and the outer tube in order to obtain a torsionally rigid structure. Additionally, a tight bond is required in order to provide a fluid seal between the stator insert and the outer tube.
The stator insert may simply be moulded inside a bare tube and be attached to the outer tube by bonding it to the inside of the outer tube using adhesives. Bonding using an adhesive, however, limits the use of the stator to an operational temperature and chemical environment required by the adhesive. Operating conditions beyond the temperature and chemical environment required may lead to the breakdown of the bond causing the stator insert to detach from the outer tube.
It is well known, in many situations where rubber or elastomer is to be connected to a rigid body, to use not only bonding, whether by natural adhesion of the elastomer or by assisted adhesion with a chemical bonding agent, but also to provide a mechanical connection. This is potentially achieved by providing re-entrants in the body to which the elastomer is being connected to achieve a mechanical interlock. It is also achieved by providing a cage or the like fixed to the body by some means, and through which cage the elastomer is moulded, also achieving a mechanical interlock, albeit indirectly through the cage, with the body. Such re-entrants could be provided in the bore of the stator tube in the case of PC pumps and motors but, of course, providing such re-entrants is problematic. Recently, there have been several designs of stators that bond mechanically to the outer tube (thereby eliminating the need for an adhesive) and U.S. Pat. No. 7,407,372, U.S. Pat. No. 7,316,548 and U.S. Pat. No. 7,329,106 disclose different methods of mechanically bonding stator inserts to inner perforated stator tubes that are connected by welding to the outer stator tube. The inner stator tubes incorporate radial apertures and are placed and welded to the outer tube prior to the injection moulding of the stator material.
Such stators are assembled by disposing an appropriate anchor element within the outer tube and permanently fixing the two together, typically using welded fixings at various locations along the length of tubes, prior to injection moulding the stator material. Furthermore, a fluid seal between the stator material and the outer stator tube is always necessary and, where no special adhesion is employed, another means is required of achieving it. In the patents just mentioned, this is, for example, achieved using sealing rings that compress the stator material following injection moulding.